The application of thin coatings of coating materials, i.e., less than about 10 microns, has become an increasingly important step in the manufacture of various products including, but not limited to; flat panel displays such as used in lap top computers, high definition television and computer cathode ray tubes; optical devices such as lenses, color filters and mirrors; hybrid circuit boards and silicon wafers and germanium wafers.
Typical methods for applying coatings to objects such as described above include, for example, dip coating and spin coating. Spin coating is often highly inefficient because typically only a small fraction of the coating material, e.g., less than about 30 percent, is actually deposited on the object to be coated. Often the remainder is wasted. Typically, dip coating methods provide an efficient use of the coating material. However, the coating thickness and coating reproducibility can be difficult to control.
One particularly useful method for applying thin coatings to objects is described in U.S. Pat. No. 4,370,376, issued Jan. 25th, 1983. At Column 1, lines 40 to 57, the patentees disclose that:
"According to the present invention, meniscus coating of an object such as a substrate is accomplished by flowing a coating material through a permeable and sloping surface, so as to develop a downward laminar flow of coating material on the outside of the sloping surface. The object, having a surface to be coated, is advanced tangentially to the downward laminar flow of coating material, such that the surface to be coated intersects the laminar flow of coating material at the apex of the sloping, permeable surface. Menisci of flowing coating material are supported both at the leading edge and the trailing edge of coating material in contact with the surface to be coated. The uniform disengagement and drainage of deposited excess coating material from the coated surface are ensured by uniform menisci and the constant downward laminar flow of coating material on the outside of the sloping surface." PA0 a) flowing the coating material in a generally upward direction through a wall of a stationary, permeable applicator to provide the coating material on an outside surface of the applicator; PA0 b) contacting the coating material on the outside surface of the applicator with the surface of the object to be coated to establish menisci of the coating material between the surface of the object and the applicator; PA0 c) advancing the surface of the object in a generally horizontal direction across the applicator; and PA0 d) maintaining the flow of the coating material through the wall of the applicator to provide a coating of the coating material on the surface.
In addition to the methods described in above cited U.S. Pat. No. 4,370,356, improved methods are desired for applying extremely thin coatings, e.g., less than 1 micron, to flat and curved planar surfaces wherein there is little variation in the coating thickness throughout the coated surface. In addition, improved methods are desired to provide thin conformal coatings to objects having irregular surfaces, such as, for example, silicon wafers which have a stepped topography.